High-speed communication of symbols over a channel can incur symbol distortion due to characteristics of the channel causing a spreading in time of the signal energy. The spreading can be such that, when a given symbol in a sequence is decoded, energy from the symbol preceding and succeeding that symbol may have leaked, or spread into the time interval of the given symbol. A result can be distortion of the given symbol that can in turn cause error in the decoding.
One conventional technique for compensating such channel conditions, and therefore reducing inter-symbol interference (ISI), is termed “de-emphasis” (sometimes alternatively referred to as “pre-emphasis” or “transmission de-emphasis”) in the transmission of the symbol. Transmission de-emphasis generally entails adding to each symbol, often called a “current symbol” or “cursor,” a weighted sum of the cursor, and one or more symbols preceding the cursor, or one or more symbols succeeding the cursor, or both. The weighted sum can be implemented as a “finite impulse response” or (FIR) filter, generally formed as a chain of delay elements or “taps,” each feeding a multiplier that applies an assigned weight or “tap coefficient.” The output of the multipliers is summed to obtain the pre-emphasis transmission signal. The tap coefficients are selected in view of objectives including the channel spreading of the symbols incurred over the distance from the transmitter to the receiver, essentially inverting the de-emphasis process.
However, channel conditions can vary. Additionally, a de-emphasis transmission signal can have higher magnitude transitions that, in some channel and signal environments, may cause interference with other signal transmission, i.e., “crosstalk.” To adapt to such changing conditions, de-emphasis filters can be configured to have multiple levels of de-emphasis. This has long posed a design conflict because conventional techniques for multi-level pre-emphasis generally prefer voltage mode drivers. However, both setting and maintaining a target impedance for voltage mode drivers can have particular design and performance costs.